[BUGFIX] Show errors on ajax file actions

Respond with status code 500 on ajax file actions, so JS can differ
between success and failure.

Resolves: #78776
Related: #69916
Releases: master, 7.6
Change-Id: I166cb86be64a26d1a3b44bfba626bba3fd151df1
Reviewed-on: https://review.typo3.org/50753


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Gleb Levitin <gleb.levitin@dkd.de>
Reviewed-by: Manuel Glauser <mail@manuelglauser.ch>
Tested-by: Manuel Glauser <mail@manuelglauser.ch>
Reviewed-by: Gianluigi Martino <gmartino27@gmail.com>
Tested-by: Gianluigi Martino <gmartino27@gmail.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>